Is protein synthesis necessary for opioid dependent associative long term potentiation in CA3 afferents?
The phenomenon of associative long term potentiation (LTP) has been observed in vivo and in vitro, while the mechanisms mediating this apparent synaptic modification process in the hippocampus remains unknown. My hypothesis focused on whether the mechanism of mossy fiber (MF) associative LTP is dependent upon protein synthesis, based on studies that show protein synthesis is needed for modifications of neuron-neuron functional activity. I began my dissertation on a study of the effects of the inhaled anesthetic isoflurane on the integrity of CA3 hippocampal afferent fiber LTP under control conditions and under the influence of the appropriate inhibitors to each pathway. The pathways I studied were the non-NMDAR dependent MF and lateral perforant pathway (LPP), and the NMDA receptor dependent medial perforant pathway (MPP) and the commissural CA3 pathway (cCA3). The results indicated that each of the pathways studied maintained their synaptic integrity under isoflurane conditions; yet the amplitude of LTP was attenuated. My second study looked at associative MF-dependent LTP with each afferent CA3 pathway studied in the previous experiment and whether this process was dependent on protein synthesis. In this experiment, the MF pathway served as the strong input which initiated LTP in the LPP, MPP, and cCA3 weak input pathways that were not able to express LTP when tetanized homosynaptically. The protein synthesis inhibitors anisomycin and cycloheximide were intracranially injected into the CA3 dendritic area prior to associative high frequency stimulation. The results of this study indicated that a novel mechanism of associative long term potentiation exists when the MF input is paired with the LPP, MPP and cCA3 pathways, regardless of the dependence of these pathways on NMDA receptors for LTP induction. I also determined that associative MF LTP is dependent on protein synthesis when it is paired with the MPP and LPP but not the cCA3 pathway. My last study determined that the transcription initiation factor Vesl-1S/Homer-1a is expressed in CA3 pyramidal cell nuclei when the MF pathway induces associative LTP with the LPP and cCA3 pathway under Ringer's conditions. Small puncta appear to be dispersed around the nuclei in the MF/LPP and MF/cCA3 associative LTP groups under cycloheximide conditions.